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81.
Zdzislaw Chilmonczyk Markus Egli Christoph Behringcr AndrS. Dreiding 《Helvetica chimica acta》1989,72(5):1095-1106
In-situ-generated phthalimidonitrene was added to five α, β-unsaturated esters containing a chiral secondary O-function at C(γ). The additions were fully suprafacial, inasmuch as the (E)-isomers 1 afforded only the trans-aziridines 2 and 3 (J(β, γ) = 4.8?5.1 Hz) and the (Z)-isomers 4 only the cis-aziridines 5 and 6 (8.2?8.5 Hz). The products 2 , 3 , 5 , and 6 where shown to possess the arabino-, xylo-, ribo-, and lyxo- configuration, respectively, by X-ray structure analysis of 2b , 2d , and 6a . The diastereoface selectivity of the nitrene additions, induced by the chiral substructure around C(γ), resulted in more 2 than 3 from 1 , but more 6 than 5 from 4 , which means that the preference of attack at the double bond switches from one side to the other depending on the C=C configuration. The preferences were higher at lower temperature. The aziridines 2a , 2d , and 3d exhibit 1H-NMR-visible isomerism at the ring N-atom; the major (78?95 %)invertomer A is always the one with the phthalimido group in trans-position to the (larger) substructure around C(γ). The other aziridines only show 1H-NMR signals of one invertomer, which – by steric reasoning - ought to be A ; this is confirmed by a 1H-NMR argument for 3a , 5a , 6a , 5c , and 6c . 相似文献
82.
Does [6+4] Cycloaddition between Pentafulvene and Cyclopentadiene Take Place? Reaction of a 1:1 mixture of cyclopentadiene (CPD) and pentafulvene ( 1a ) at 20° gives a complex mixture. The low-molecular-weight part mainly consists of pure and mixed dimers (ca. 73 %) besides corresponding trimers (ca. 20%) and some corresponding oligomers according to GC/MS investigations (Fig. 1). The 3 predominant ‘mixed dimers’ between CPD and 1a have been separated, and structures 4 – 6 (Scheme 3) are assigned according to 400- and 600-MHz 1H-NMR investigations. These results show that HOMO(CPD)-LUMO(fulvene) interactions are important in pentafulvene cycloadditions. Dimer 6 results from [6+4] cycloaddition followed by [1,5]-H shifts. 相似文献
83.
The mechanism of biological dinitrogen reduction is still unsolved, and the structure of the biological reaction center, the FeMo cofactor with its seven iron atoms bridged by sulfur atoms, is too complicated for direct attack by current sophisticated quantum chemical methods. Therefore, iron-sulfur complexes with biologically compatible ligands are utilized as models for studying particular features of the reduction process: coordination energetics, thermodynamic stability of intermediates, relative stability of isomers of N2H2, end-on versus side-on binding of N2, and the role of states of different multiplicity at a single iron center. From the thermodynamical point of view, the crucial steps are dinitrogen binding and reduction to diazene, while especially the reduction of hydrazine to ammonia is not affected by the transition metal complex, because the complex-free reduction reaction is equally favored. Moreover, the abstraction of coordinated ammonia can be easily achieved and the complex is recovered for the next reduction cycle. Our results are discussed in the light of studies on various model systems in order to identify common features and to arrive at conclusions which are of importance for the biological mechanism. 相似文献
84.
A high-performance liquid chromatography-mass spectrometry method has been developed for the simultaneous determination of native carnitine and eight acylcarnitines in urine. The procedure uses a solid-phase extraction on a cation-exchange column and the separation is performed without derivatization within 17 min on a reversed-phase C8 column in the presence of a volatile ion-pairing reagent. The detector was an ion trap mass spectrometer and quantification was carried out in the MS-MS mode. Validation was done for aqueous standards at ranges between 0.75 and 200 micromol/l, depending on the compound. Carnitine was quantified in urine and comparison with a radioenzymatic assay gave a satisfactory correlation (R2 = 0.981). The assay could be successfully applied to the diagnostic of pathological acylcarnitines profile of metabolic disorders in urines of patients suffering from different organic acidurias. 相似文献
85.
Genetic algorithms (GA) were used to solve one of the multidimensional problems in computational chemistry, the optimization of force field parameters. The correlation between the composition of the GA, its parameters (p(c), p(m)) and the quality of the results were investigated. The composition was studied for all combinations of a Simple GA/Steady State GA with a Roulette Wheel/Tournament Selector using different values each for crossover (0.5, 0.7, 0.9) and mutation rates (0.01, 0.02, 0.05, 0.10, 0.20). The results show that the performance is strongly dependent on the GA scheme, where the Simple GA/Tournament Selector yields the best results. Two new MM3 parameters were introduced for rhenium compounds with coordination number four (204) and coordination number five (205), the formal oxidation states of rhenium ranging from +V to +VII. A manifold of parameters (Re-C, N, O, S) was obtained by using a diverse set of CSD structures. The advantage of the GA vs. UFF calculations is shown by comparison of several examples. The GA optimized parameters were able to reproduce the geometrical data of the X-ray structures. 相似文献
86.
Luigi Fabbrizzi Piero Paoletti Markus C. Zobrist Gerold Schwarzenbach 《Helvetica chimica acta》1973,56(2):670-680
The thermodynamic functions of the proton transfer H2tn2++tn → 2 Htn+ (tn = 1,3-diaminopropane) have been determined in aqueous solutions containing different amounts of KCl (0.05 ? μ ? 3.01). The free energy (?ΔG) of the process decreases, whereas the enthalpy (-ΔH) increases with μ. There is reason to believe that the reaction is entirely controlled by the Coulomb forces between the two protonic charges. The electrostatic energy involved can be described in terms of a model incorporating an effective dielectric constant εe, such that δεe/δμ and δ2εe/δμδT are both positive. The polarisation of pure water is produced by orientation of hydrogen-bonded dipole molecules H2O, whereas the electrolyte solution is polarised in addition by dislocation of the ions K+ and Cl?. Our results demonstrate that the former type of polarisation is much more temperature dependent than the latter. 相似文献
87.
Siegers C Biesalski M Haag R 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(11):2831-2838
Highly protein-resistant, self-assembled monolayers (SAMs) of dendritic polyglycerols (PGs) on gold can easily be obtained by simple chemical modification of these readily available polymers with a surface-active disulfide linker group. Several disulfide-functionalized PGs were synthesized by N,N'-dicyclohexylcarbodiimide-mediated ester coupling of thioctic acid. Monolayers of the disulfide-functionalized PG derivatives spontaneously form on a semitransparent gold surface and effectively prevent the adsorption of proteins, as demonstrated by surface plasmon resonance (SPR) kinetic measurements. A structure-activity relationship relating the polymer architecture to its ability to effectuate protein resistance has been derived from results of different surface characterization techniques (SPR, attenuated total reflectance infrared (ATR-IR), and contact-angle measurements). Dendritic PGs combine the characteristic structural features of several highly protein-resistant surfaces: a highly flexible aliphatic polyether, hydrophilic surface groups, and a highly branched architecture. PG monolayers are as protein resistant as poly(ethylene glycol) (PEG) SAMs and are significantly better than dextran-coated surfaces, which are currently used as the background for SPR spectroscopy. Due to the higher thermal and oxidative stability of the bulk PG as compared to the PEG and the easy accessibility of these materials, dendritic polyglycerols are novel and promising candidates as surface coatings for biomedical applications. 相似文献
88.
Markus Bergauer 《Tetrahedron》2004,60(5):1197-1204
Chemo- and regioselective transformations of asparagine gave access to optically active 5- and 6-amino tetrahydroindolizines when the 3-aminobutyrolactone (S)-2 was employed as a key intermediate. The target compounds were approached by a sequential and regiocontrolled bis-electrophilic attack in the positions 2 and 3 of the pyrrole ring system. Receptor binding experiments showed stereocontrolled receptor recognition leading to the D3 selective agonist (S)-8 with D3 binding that is comparable to the natural neurotransmitter dopamine. 相似文献
89.
90.
Summary. A series of novel tridentate ligands with nitrogen and oxygen donor sites was synthesized starting from enantiomerically
pure (S)- and (R)-1-(pyridin-2-yl)ethylamine, the preparation and resolution of which was developed. The new optically active ligands were
tested as in situ catalysts together with Ru(PPh3)3Cl2 in the enantioselective transfer hydrogenation of acetophenone with isopropanol. The secondary amine ligand (S)-2,4-di-tert-butyl-6-(1-(pyridin-2-yl)ethylamino)methylphenol gave the best results with almost quantitative conversion and 47%ee.
Received August 17, 2001. Accepted August 27, 2001 相似文献